In recent years, transparent substrates such as glass, plastics, etc. are often intended to be rendered antistatic by imparting conductive properties to the transparent substrate surface.
To impart conductive properties to the substrates, the coating materials, having been rendered conductive by the incorporation of carbon powder, metallic powder, carbon fiber or metallic fiber therein, are coated on the substrate surface. The above-mentioned procedure, however, involves such problems that because the coatings formed on the substrate surface from the coating materials referred to above assume a gray or black color, the color tone of the thus coated substrate is marred. This is because, the carbon powder, metallic powder, carbon fiber or metallic fiber incorporated into the coating materials absorbs light that permeates the substrates through said coating materials.
Furthermore, in order to impart conductive properties to the substrates, there has widely been practiced a process which comprises forming conductive coatings on the substrate surface by coating said substrate with conductive coating materials prepared by dispersing or dissolving conductive particles and binder resins in organic solvents. In this process, tin oxide, indium oxide or the like has been used as conductive particles to form conductive coatings excellent in transparency.
However, when transparent conductive coatings are intended to be formed by the use of such conductive oxides as tin oxide, indium oxide and the like as conductive particles, the transparency of the resulting conductive coatings is not always found satisfactory.
In order to solve such a problem as associated with the transparency of transparent conductive coatings, Japanese Patent Publication No. 9343/1986 discloses conductive coatings obtained by the use of conductive particles consisting of tin oxide containing antimony and having an average particle diameter of less than 0.2 .mu.m which is smaller than the wavelength of visible light.
For preparing the conductive coating materials disclosed in Japenese Patent Publication No. 9343/1986, however, the conductive particles having an average particle diameter of less than 0.2 .mu.m are prepared by pouring an aqueous solution having dissolved therein specific amounts of tin chloride and antimony chloride into hot water to hydrolyze said chlorides, separating the deposited sediment by filtration, rinsing the separated sediment and then subjecting said sediment to heat treatment. On that account, there has been brought about such problems as will be mentioned hereinafter. That is, as disclosed in the above-mentioned publication, when precursors of conductive fine particles are deposited in the solution by hydrolysis of the halides, the sediment obtained is in a state where very fine primary particles are aggregated. On that account, fine particles obtained by separating the sediment by filtration from the solution, rinsing the separated sediment and then subjecting the rinsed sediment to heat treatment are in a state where the primary particles have been sintered, having a broad particle size distribution in general.
Accordingly, when the conductive fine particles thus obtained are incorporated and dispersed in plastics or coating materials, the sintered fine particles must be pulverized in order to perform the incorporation and dispersion of the fine particles homogeneously. However, the grinding of such conductive fine particles as prepared through the above-mentioned sediment forming step is not always easy because they are very small in particle diameter, high in surface activity, strong in bonding strength among particles, and the sintering of the fine particles proceeds at the time of heat treatment thereof. In addition thereto, even when the such fine particles as already sintered are pulverized to fine particles so as to reduce in particle diameter, conductive fine particles having a sufficiently sharp particle size distribution cannot be obtained. Moreover, such pulverized particles have inherently strong aggregation power and hence relatively large amounts of dispersant must be used to prevent the particles from aggregating again.